This invention relates to an apparatus for lowering the temperature of articles so as to cool, supercool or freeze the articles, the apparatus being used for example, to freeze foods and to separate lenses from the lense supporting device, which is used while the lenses are polished, by cooling or supercooling them.
Prior art apparatuses of this kind have in common a low temperature box within which the temperature is kept at a predetermined low value so that the articles introduced into the box are cooled to the predetermined low temperature. The following two ways are used to lower the temperature in the box.
The first way is the so called "air-blast cooling or freezing method" in which the air in the box is forcibly circulated through a heat exchanger so that the air cooled or supercooled by the heat exchanger is directed to the articles. However, the heat exchanger can lower the temperature of the air to only about -50.degree. C. at most, and thus this method is not suitable for articles which must be rapidly cooled, supercooled or frozen. For example, when foods which are at a room temperature or higher are frozen, the following problem occurs. That is, when the temperature of the food drops just a little below 0.degree. C., it goes into a "zone of maximum ice crystal formation" in which the temperature of the food does not go down substantially further for a certain time even though the temperature of the air around them is kept at a low degree relative to the food. However, to obtain satisfactory frozen food, it is required that the time in such zone of maximum ice crystal formation be made as short as possible. However, with the air-blast method described above, the temperature difference between the food and the air around it is about -50.degree. C. at most, so this limits the shortening of this time period.
In the second way, a liquefied gas is sprayed in to the low temperature box. This method can lower the temperature of the air in the box to about -120.degree. C. or lower. Thus, this method can overcome the problem existing in the air-blast method as described above. However, this method involves other problems. That is, the low temperature box has a relatively large space so that a large number of article can be treated at once and thus a large quantity of liquefied gas is required to lower the temperature of the air in the box. However, liquefied gas is expensive, so the operating cost is much higher than that of the air-blast method. Further, although the greater the temperature difference between the food and the air around it, the greater the rate of freezing the food, the acceleration of the rate decreases as the temperature difference becomes greater, so that it is not efficient to freeze the food at such a very low temperature from the beginning to the end of the freezing operation. Furthermore, in connection with frozen food, although rapid freezing is required when the temperature of the food falls into the "zone of maximum ice crystal formation" so as to shorten the time spent in the zone to improve the quality of the frozen food, the quality does not have any substantial relation with the freezing rate at temperatures of the food other than that in the "zone of maximum ice crystal formation". Accordingly, it is not necessary to place the food in the very low temperature atmosphere as noted above throughout the freezing operation. Further, the surface of certain kinds of articles tends to crack upon rapid cooling, supercooling or freezing.